1. Field
Modulating an extreme ultraviolet (EUV) light.
2. Background
The extreme ultraviolet (EUV) light commonly refers to the region of spectrum having a wavelength of approximately 13.5 nm. One application of the EUV is in photolithography where reduced feature size (the critical dimension) of a circuit is desired. The EUV may be used for patterning; that is, creating a circuit design by projecting the EUV light on a wafer covered by a mask. The mask blocks the EUV light from entering the covered area. Thus, selective areas of the wafer may be etched according to the circuit design.
As the EUV is highly reflective, conventional system including lens and quartz cannot be used to direct the projection of the EUV light. Thus, EUV mirrors are generally used. Presently, EUV mirrors are made of machined flat materials, usually diamond turned, which are coated with a material with a high index of refraction at 13.5 nm. Ruthenium is commonly used as a coating material, as are stacks of materials that have alternating high and low refractive indices in the EUV region of the spectrum. Examples are MoRu and Be multilayers, which has been disclosed by, for example, J. F. Seely et al. in the article “High-Efficiency MoRu—Be Multilayer-Coated Gratings Operating near Normal Incidence in the 11.1–12.0 nm Wavelength Range,” published by Applied Optics, vol. 40, No. 31, pp. 5565–5574. The EUV mirrors are typically used at grazing incidence to enhance the reflection coefficient.
In photolithography, the EUV may be used not only to pattern a wafer with a mask, but also to write the mask. EUV light beams may be modulated such that selective portions of the beams may be projected onto a wafer while the rest of the beams may be directed away from the wafer. As such, the projected beams form a mask pattern on the wafer. Due to the short wavelength, EUV light beams cannot be easily modulated. Conventional mechanical shutters typically have slow modulation speed and therefore do not achieve good performance.